Display device with pre-charging arrangement

ABSTRACT

The invention relates to a display device comprising a plurality of light emitting elements ( 1 ), wherein at least one of the elements has an associated capacitor (C 1 ). The display device comprises pre-charging means ( 7;8 ) for generating a pre-charge signal for charging the associated capacitor (C 1 ) at least partly. The pre-charging means ( 7;8 ) are adapted for generating a pre-charge signal comprising at least a first pre-charge signal in a first pre-charge stage and a second pre-charge signal in a second pre-charge stage. The pre-charge signal preferably comprises a current pre-charge signal followed by a voltage pre-charge signal. The combined pre-charge signal has the advantage of fast, but accurate pre-charging.

The invention relates to a display device comprising a plurality oflight emitting elements, at least one of the elements having anassociated capacitor, the device comprising pre-charging means forgenerating a pre-charge signal for charging the associated capacitor atleast partly.

In more and more display applications, light emitting matrix displays,such as organic light emitting displays or inorganic light emittingdisplays, are used. The basic device structure of a light emittingmatrix display essentially comprises a structured electrode or anode, acounter electrode or cathode and a light emitting layer, sandwichedbetween the anode and the cathode. In a passive matrix display the anodemay comprise a set of separate parallel anode strips, also referred toas anode columns (or anode rows depending on their direction), eachbeing adapted to be connected to a current or voltage source. Further,the cathode may comprise a set of separate parallel cathode strips, alsoreferred to as cathode rows (or cathode columns depending on theirdirection), their direction usually being essentially perpendicular tothe anode strips or columns. The point of intersection of such an anodeand cathode essentially defines a pixel or light emitting element ofsaid display device, and said pattern of anodes and cathodes hencedefines a matrix of pixels. An electrical representation of such apassive matrix display is provided in FIG. 1. Light emitting elementsare indicated as diodes 1. Such a passive matrix display may beaddressed line by line, by applying subsequent pulses, here indicated assignals 3, to subsequent lines 2. The lines are indicated by means ofreference numeral 2 in FIG. 1 and are here represented as a commoncathode, the cathodes being selected one by one together with all anodesin a column 4. The anodes are supplied with a current (signals 5) of anenergy corresponding to the grey value required. Grey values are usuallyobtained by setting the amplitude of the current or the on-time of thecurrent source according to the conditions required.

The light emitting elements may be driven by a voltage or by a current.Current driven matrix displays, wherein a forward current is drawnthrough the light emitting element 1, have several advantages. The mainadvantage of current driving of such a matrix display is a good greyscale control. A light emitting element 1 will essentially generatelight when a forward current is drawn through the light emitting layer,the current being applied by said anode/cathode pattern via columns 4.The light originates from electron/hole pairs recombining in the activearea, with the excess energy partly being emitted as photons, i.e.light. The number of photons generated (i.e. the brightness of thepixel) depends on the number of electrons/holes injected in the activearea, that is the current flowing through the pixel.

A disadvantage of current driving is that an additional pre-chargedriver is needed to charge parasitic capacitors present in the displaymatrix device. FIG. 2 shows an equivalent circuit for a passive matrixdisplay. The display is current driven by current sources 6. Line or rowselection is obtained from voltage sources 7. As illustrated by theblack coloured diodes 1, these diodes are selected by the voltage source7 by applying a low voltage, for example, a ground level voltage to theselected row; to the other rows a high voltage, indicated by means of +,is applied which effectively blocks all diodes attached to the otherrows. The black colored diodes 1 are driven by the respective currentsource 6, i.e. the light emitting element 1 generates light. It is wellknown that e.g. a light emitting element such as a diode 1 has anassociated capacitor C1, resulting e.g. from a parasitic capacitancecaused by the sandwich structure referred to above and/or from theconnection leads within and outside the display device. This associatedcapacitor has to be charged. Moreover, associated resistances R may bepresent, originating from the anode and cathode structures andconnections in the display device.

U.S. Pat. No. 5,723,950 discloses a pre-charge driver for light emittingdevices with an associated capacitance. A square wave of current fordriving the light emitting device is initially applied together with asharp current pulse to rapidly charge the associated capacitor of thelight emitting device. Such an approach is colloquially referred to ascurrent boosting, which expression is used in the present text as anequivalent for current pre-charging.

However, current boosting, although successful in rapidly pre-chargingthe associated capacitor, has some drawbacks. These drawbacks relate,amongst other things, to inflexibility, inaccuracy and/orcost-ineffectiveness if current boosting according to the prior art isapplied.

It is an object of the invention to provide a display device withimproved pre-charging means. The invention is defined by the independentclaims. The dependent claims define advantageous embodiments.

The object is achieved by providing a display device characterised inthat said pre-charging means are adapted for generating said pre-chargesignal comprising at least a first pre-charge signal in a firstpre-charge stage and a second pre-charge signal in a second pre-chargestage. By dividing the pre-charge stage into several sub-stages (i.e.the first, second and further pre-charge stages), a higher degree offlexibility of the pre-charging of the associated capacitor can beachieved, since it becomes possible to provide a pre-charging signalsatisfying several different pre-charging criteria during pre-charging.These pre-charging criteria may refer to accuracy in the resultingsignals and/or to the time wherein pre-charging of an associatedcapacitor is achieved.

It should be appreciated that the invention applies to all displaydevices wherein an associated capacitor is to be charged. Besides thecurrent driven passive matrix displays, small molecule or polymerorganic LED displays, inorganic displays, electroluminescence displays,field emission displays, also active-addressed displays and liquidcrystal displays (LCD's) may benefit from a pre-charging arrangement asdisclosed. The method proposed here can be advantageously used indisplays where a fast preset is required while keeping the chargingcurrents limited. As the dimensions of the display pixels need not befixed, the method can be used as well for driving segmented displays.Below an example for a current driven passive matrix display will bediscussed in detail.

In an embodiment of the invention the pre-charging means comprise acurrent source for generating a current pre-charge signal during saidfirst pre-charge stage and a voltage source for generating a subsequentvoltage pre-charge signal during said second pre-charge stage. Thisembodiment of combined boosting has the advantage that the rapidcharging of the current boosting approach is combined with the lessrapid, but much more accurate, subsequent voltage boosting. First theassociated capacitor is pre-charged to roughly the operating voltage ofthe light emitting element and subsequently a pre-charge voltage isapplied that may accurately approach the operating voltage, which is thevoltage needed to drive the display diode(s) at the required luminancelevel. Moreover, the current boost has to be less accurate in comparisonwith pure current boosting, since a more accurate pre-charge signal isapplied afterwards by a voltage boost. Therefore, the means for applyingthe current pre-charge signal have to fulfil less severe requirements asa consequence of which the current boost source can be implemented inthe display device more easily and less costly.

In an embodiment of the invention the pre-charge current is limited.High pre-charge currents may cause interference in the display device,as a result of which light emitting elements that are not driven maygenerate light. Moreover high pre-charge currents may cause high voltagedrops across parasitic resistances, drawn as resistances R in FIG. 2, inthe display device. Limitation of the pre-charge current is preferablyachieved by using a current source, which source may be connected to avoltage source adapted for selecting a light emitting element in amatrix of elements during operation. The latter arrangement provides theadvantage of automatic saturation of the pre-charge current and easyimplementation in the display device. The current may also be limited bya resistance or a combination of resistances that can be selected inorder to obtain an appropriate pre-charge current. It should beappreciated that alternative current limiting elements, such as e.g.coils, may be used alternatively or additionally.

In an embodiment of the invention the pre-charging means comprises avoltage source in order to generate a voltage pre-charge signal via afirst resistance during said first pre-charge stage and a subsequentvoltage pre-charge signal via a second resistance during said secondpre-charge stage. Such an approach may reduce the disadvantage of singlevoltage boosting and can be very easily implemented in the displaydevice. Since an accurate current source is no longer needed, thisapproach is very cost-effective as well.

In an embodiment of the invention the pre-charging means is adapted toobtain the operating voltage of at least one light emitting element andto generate during the second pre-charge stage a pre-charge voltagesignal in accordance with said operating voltage. This embodimentprovides the advantage that automatic adaptation is achieved forvariations in capacitance of the associated capacitors and in thematerial of the light emitting elements. Variation may be due to ageingof the elements, and/or to the fact that the organic materials may haveslightly different properties for different batches and/or to variationsin layer thickness. Preferably, the operating voltage is obtained in asteady state of the light emitting element, i.e. near the end of thetime during which the element is driven. Moreover, there is no need toset the pre-charge current amplitude and time for every brightness levelas is the case for pure current pre-charging schemes. Further, a uniformbrightness is obtained, especially at low grey levels, since the amountof charge required for generating these low grey levels is smallcompared to the charge charging the associated capacitor(s).

The invention also relates to an electroluminescent matrix pre-chargingarrangement comprising the features with respect to the pre-chargingsignal and the pre-charging means as discussed above.

The invention also relates to an electronic device comprising such adisplay device and/or pre-charging arrangement. Such an electronicdevice may e.g. be a device such as a monitor and also a handheld devicesuch as a mobile phone or a PDA. Also multiplexed segmented displays areadvantageously driven according to the invention, especially when thedimensions or materials of the various segments are different.

U.S. Pat. No. 6,369,786 B1 discloses a matrix of display elementswherein voltage boosting is applied up to a threshold voltage. However,neither a preceding current boosting nor voltage boosting to theoperating voltage is disclosed.

These and other aspect of the invention will be apparent from anddescribed in more detail below with refrence to the attached drawings,in which:

FIG. 1 shows a passive matrix organic LED display in a common cathodeconcept;

FIG. 2 shows an equivalent circuit for a part of the passive matrixdisplay of FIG. 1;

FIGS. 3A and B illustrate the conventional current boosting approach fora LED display;

FIGS. 4A and B illustrate the conventional voltage boosting approach fora LED display;

FIGS. 5A and B show a first embodiment according to the invention ofcombined current and voltage boosting;

FIGS. 6A and B show a second embodiment according to the invention ofcombined current and voltage boosting;

FIGS. 7A and B show a third embodiment according to the invention ofvoltage boosting in two stages.

For an adequate understanding of the embodiments of the invention, firstthe concepts of current boosting and voltage boosting will be brieflydiscussed.

FIG. 3A shows a single light emitting diode 1, hereinafter referred toas LED 1, which is part of a passive matrix display as depicted inFIG. 1. LED 1 is current driven by current source 6 and can be selectedin the passive matrix by voltage source 7. A capacitance C1, directlyassociated with LED 1, is shown together with the capacitance C_(n)representing all associated capacitors of the LEDs 1 in column 4 to becharged. For pre-charging the associated capacitors C1 and C_(n), acurrent boost source 8 is provided. Moreover, the circuit exhibitsswitches S1, S2, S3, S4 and S5, for connecting the LED 1 to the currentsource 6, the voltage source 7 and the current boost source 8.

In FIG. 3B a current boost scheme is shown with respect to FIG. 3A. Thegraphs shown represent the current I as a function of time t, indicatedin FIG. 3A, and the voltage V at point X. The bottom graph refers to thelight L emitted by the LED 1. Suppose that LED 1 is required to generatelight in the passive matrix display at time t=t₀. Since the associatedcapacitors C1 and C_(n) are charged before a driving current I_(d) flowsthrough the LED 1, a current preceding the drive current is required tocharge these associated capacitors. This current is typically providedas a boost current I_(b). This boost current I_(b) is obtained from theboost current source 8 at a suitable time t before t₀, for example,between t=0 and t=t₀. Boost current I_(b) typically is significantlyhigher than the driving current I_(d) for driving the LED 1 from thecurrent source 6.

At t=0 switches S2, S3 and S4 are open, while S1 and S5 are closed. Inthis situation LED 1 is not selected and the current boost I_(b) maycharge up the associated capacitors C1 and C_(n). The boost currentI_(b) is supposed to be the maximum allowed current, which can be set byprogramming the current amplitude and time. In this way the voltage Vover the LED 1 can be boosted rapidly to a particular voltage level,which can be chosen close to the operating voltage. As the final voltageover the LED 1 generated by boosting is reached by programming thecurrent amplitude and time, a non-optimal boost may result from anyvariation in the associated capacitors. This variation may e.g. becaused by layer thickness variations in the LED sandwich structure,material ageing, or properties of the interconnecting leads. The finalvoltage also depends on the timing and amplitude of the boost currentI_(b). As a result this final voltage is defined less accurately, andmay even exceed the operating voltage, i.e. overshoot may occur.

At t=t₀ switch S1 is opened, i.e. LED 1 is selected in the passivematrix display. Moreover S4 and S5 are opened, while S2 and S3 areclosed so as to drive the LED 1 from the current source 6 with thedriving current I_(d). As shown by way of example in FIG. 3B, thevoltage V over the diode at t=t₀ is not accurate in that the operatingvoltage is not yet reached at that time and therefore the light Lgenerated from the LED 1 is not yet at the required level L_(d). Alsosome initial overshoot (not shown) may be present.

In conclusion, current boosting provides a fast, but inaccurate way topre-charge the associated capacitors of a passive matrix display.

In FIG. 4A, a voltage boosting scheme is shown. Components equivalent tothose depicted in FIG. 3A for the current boosting scheme are indicatedby identical reference numbers. In this example, the voltage boostscheme applies the voltage source 7 for selecting a LED 1 of the passivematrix display as well as for the voltage boost, employing switch S6.

FIG. 4B shows a voltage boosting scheme to be executed by the circuitdepicted in FIG. 4A. Just before time t=0, switch S4 may be closed toguarantee that all charge at point X has been removed, and no pixelcontent related cross-talk will occur, thereafter S4 is opened.

At time t=0 (when S1 and S6 are closed) the voltage of voltage source 7is applied to LED 1, which theoretically results in an infinitely highcurrent I. The final voltage across the LED 1 as result of the voltageboosting is accurately obtained before time t=t₀. At time t=t₀ S2 and S3are closed and the light L emitted from the LED 1 the required levelL_(d) has from time t=t₀ onwards, as can be seen in FIG. 4B.

In a voltage boosted system, the final voltage is fixed by the requiredvalue of the voltage V across the LED 1, independent of the value of aseries resistance in the current loop formed by the voltage source 7,the associated capacitors C1, Cn and their interconnections. A seriesresistance limits the current. The voltage source is not an idealvoltage source and further parasitic column and row resistances arepresent, resulting from the electrodes and the connections to theseelectrodes of the passive matrix display device. This resistance sets aminimum charging time, e.g. about 3 times the RC time constant, beforethe associated capacitors C1, Cn are properly charged. As the resistancecan be large, a significant time delay can be the result of this. Thus,although the voltage obtained at t=t₀ is accurate, a time penalty ispresent in the voltage boosting scheme.

In conclusion, voltage boosting provides an accurate, but slow way topre-charge the associated capacitors of a passive matrix display andlarge initial currents may flow.

FIG. 5A shows a boosting and driving circuit according to a firstembodiment of the invention. In FIG. 5A components identical to thoseshown in FIG. 3A and FIG. 4A are indicated by identical reference signs.

Current source 6 can be connected to the anode of LED 1 via switch S3 todrive this LED 1. The anode can be further connected to ground potentialvia switch S4. A (low-ohmic) voltage source 7 is adapted to provide apotential to the cathode of LED 1 via switch S1 in order to select LED 1in a passive matrix display. If S1 is closed, LED 1 is not selected andwill not generate light. The cathode of LED 1 may be further connectedto ground potential via switch S2. LED 1 further has an associatedcapacitor C1, in parallel with LED 1. Moreover an associated capacitorC_(n) is present, parallel to LED 1, representing the associatedcapacitors of the n other light emitting elements in the same anodecolumn 4 and the parasitic line capacitance. A current boost source 8can be connected to the anode of LED 1 via switch S5. Current source 6and current boost source 8 are supplied by a supply voltage V_(s).Moreover voltage source 7 can be connected via switch S6 to the anode ofLED 1. Finally via lead 9 and sensing unit 10, the voltage source 7 isenabled to sense or measure the potential of point X, i.e. the voltageapplied over the LED 1 if S2 is closed.

In FIG. 5B a boosting and driving scheme is depicted in order toillustrate the operation of the first embodiment according to theinvention.

At time t=0, switches S1 and S5 are closed, i.e. the LED 1 is notselected in the passive matrix display and a boost current I_(b) isapplied via the current boost source 8 as a first pre-charge signal tocharge up the associated capacitors C1 and C_(n). The limits for I_(b)are set by the requirements of avoiding cross-talk in the displaydevice, while providing enough charge to charge up the associatedcapacitors. During this first stage, the voltage over the LED 1 isroughly and rapidly brought to a level near the operating voltage forthe LED 1.

If this voltage is reached, a second boost stage is initiated at timet=t_(s), closing switches S2 and S6, wherein a subsequent voltage boostis applied as a second pre-charge signal. During this second stage thevoltage over the LED 1 is accurately brought to the operating voltage.The voltage supplied is preferably equal to the operating voltage in thesteady state of LED 1, i.e. the state at the end of selection of theline by voltage source 7. During this second stage only very smallcurrents are required to bring the voltage across the LED 1 to the levelof the operating voltage. The voltage across the LED 1 can be sensed ormeasured via connection 9 and fed back to the voltage source 7. Thesensing unit 10 of the LED voltage V enables an overshoot of the voltageover the diode during the first pre-charge stage, resulting from therough current boost, to be corrected in the second pre-charge stage, asillustrated in FIG. 5B by the dashed line.

At time t=t₀, switches S2 and S3 are closed and the LED 1 is ready toreceive the driving current Id and emit the required amount of lightL_(d). Preferably all the associated capacitors C1 and C_(n) are chargedup completely before LED 1 is selected by opening switch S1 and closingswitch S2. Other switching sequences are possible, e.g. selecting LED 1by opening switch S1 at the time of transition between the firstpre-charge stage and the second pre-charge stage.

In conclusion, by combining the concepts of current boosting andsubsequent voltage boosting the advantages of both concepts can beachieved, i.e. a rapid and accurate boosting scheme, while the maximumcharging currents are limited to avoid cross talk. Moreover, the currentboost has to be less accurate in comparison with pure current boosting,since a more accurate pre-charge signal is applied afterwards in theform of a voltage boost. Therefore, the circuitry for applying thecurrent pre-charge signal has to fulfill less severe requirements and asa consequence the current boost source can be implemented in the displaydevice more easily and less costly.

In FIG. 6A a second embodiment of the invention is shown. In FIG. 5Acurrent boost source 8 was supplied with a high potential from a supplyvoltage V_(s), The combined boosting circuit depicted in FIG. 6A isequivalent to the circuit depicted in FIG. 5A, except for the lead 11connecting the current boost source 8 to the voltage source 7. Thisset-up can be easily implemented in an integrated circuit for drivingthe passive matrix display. Another advantage of this set-up is that themaximum boost current does not have to be accurately programmed inadvance.

A sensing unit 10 may be employed for accurately adapting the voltage ofthe voltage source 7.

In operation, as displayed in FIG. 6B, during a first pre-charge stagestarting at time t=0, a boost current I_(b) is applied from the currentboost source 8 by closing switches S1 and S5. As the current charges theassociated capacitors C1 and C_(n), the voltage V across the LED 1increases. When the potential of point X approaches the potentialsupplied from voltage source 7, the current boost source 8 can no longersupply the initial boost current I_(b). This can be seen in FIG. 6B asthe current I decreases when the time t approaches time t_(s).

At time t=t_(s), the current I drops rapidly and the second stage isinitiated. In this second stage, switch S6 closes, thereby applying asubsequent voltage boost from the voltage source 7 to LED 1. The voltageis brought accurately to the operating voltage before time t=t₀.

At time t=t₀, switches S2 and S3 are closed to operate the LED 1.

In the embodiments discussed above, limitation of the boost currentI_(b) was achieved by supplying the boost current from a current boostsource 8. However, limitation of the boost current can also be achievedby using one or more resistances in combination with a voltage source.Such an embodiment is shown in FIG. 7A. In this embodiment, tworesistances R1 and R2 are employed. R1 has a resistance value that issignificantly larger than R2. It is appreciated that more resistors orcombinations of resistors can be employed as well. The resistors can beselected by switches S7 and/or S8. Moreover it will be appreciated thatthe resistance may result from other components as well, such as theresistances intrinsic to the switches S7 and S8 or coils. The provisionof an arrangement of resistances increases the flexibility to apply aboost current I_(b) just below the maximum allowed current.

FIG. 7B illustrates the operation of the set-up shown in FIG. 7A.

At time t=0 the first pre-charge stage is started by closing switches S1and S7. A voltage from the voltage source 7 is applied via theresistance R1 to LED 1. By using a proper value for R1, the currentflowing in the display device can be limited.

At time t=t_(s), resistance R2 is employed by closing switch S8 and thesecond pre-charge stage is initiated. Note that S7 may remain closed, asthis decreases the overall resistance to below R2. This second stage ispreferably entered while the current I in the first stage decreasesrapidly, as is the case near time t=t_(s) here.

At time t=t₀, switches S2 and S3 are closed to operate the LED 1.

In the embodiment of FIG. 7A two voltage boosting stages are employedvia the resistances R1 and R2. The advantage of the boosting and drivingcircuit depicted in FIG. 7A is that no accurate current source isneeded, as a result of which a very cost-effective circuit is obtained.Fast voltage boosting is obtained here in that, as the currentdecreases, a lower resistance is selected as a result of which duringthe second discharge phase a higher current is obtained for fastcharging of the associated capacitors. The speed of charging is thusdetermined by the choice of the resistors R1 and R2. More resistor orswitch sections may be added e.g. to increase flexibility.

For the purpose of teaching the invention, preferred embodiments of thedisplay device, the pre-charging arrangement and the electronic devicecomprising such a display device have been described above.

It should be noted that the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims. In the claims, any reference signsplaced between parentheses shall not be construed as limiting the claim.The word “comprising” does not exclude the presence of elements or stepsother than those listed in a claim. The word “a” or “an” preceding anelement does not exclude the presence of a plurality of such elements.In the device claim enumerating several means, several of these meanscan be embodied by one and the same item of hardware. The mere fact thatcertain measures are recited in mutually different dependent claims doesnot indicate that a combination of these measures cannot be used toadvantage.

1. A display device comprising a plurality of light emitting elements atleast one of said elements having an associated capacitor, said devicecomprising pre-charging means for generating a pre-charge signal for atleast partially charging said associated capacitor, said pre-chargesignal comprising at least a first pre-charge signal in a firstpre-charge stage and a second pre-charge signal in a second pre-chargestage, wherein said pre-charging means comprise a current source forgenerating a pre-charge current as the first pre-charge signal duringsaid first pre-charge stage, and a voltage source for generating asubsequent pre-charge voltage subsequent to the pre-charge current asthe second pre-charge signal during said second pre-charge stage.
 2. Thedisplay device according to claim 1, wherein a current limiting means isprovided, which is adapted to limit said pre-charge current inoperation.
 3. The display device according to claim 2, wherein saidcurrent limiting means is said current source.
 4. The display deviceaccording to claim 2, wherein said current limiting means comprises atleast one resistor arranged so as to limit said pre-charge current. 5.The display device according to claim 1, wherein said voltage source isadapted to select, in operation, at least one of said light emittingelements and said current source is connected to said voltage source soas to limit the pre-charge current.
 6. The display device according toclaim 1, wherein said voltage source is configured to generate a firstpre-charge voltage as the first pre-charge signal during said firstpre-charge stage and second pre-charge voltage as the second pre-chargesignal subsequent to the first pre-charge voltage during said secondpre-charge stage.
 7. The diplay device according to claim 6, wherein thedisplay device comprises means for selecting a resistance to generatesaid first pre-charge voltage and said subsequent second pre-chargevoltage.
 8. The display device according to claim 1, wherein a sensingunit is provided to obtain an operating voltage of at least one lightemitting element and said voltage source is adapted to generate saidsubsequent pre-charge voltage in accordance with said operating voltage.9. The display device according to claim 8, wherein said operatingvoltage is obtained by said sensing unit in a steady state of said lightemitting element.
 10. A pre-charging arrangement for pre-charging atleast one capacitor associated with at least one light emitting elementof a display device, said pre-charging arrangement being adapted forgenerating a pre-charge signal comprising at least a first pre-chargesignal in a first pre-charge stage and a second pre-charge signal in asecond pre-charge stage, wherein the first pre-charge signal is providedby a current source as a pre-charge current, and the second pre-chargesignal is provided by a voltage source for generating a subsequentpre-charge voltage subsequent to the pre-charge current as the secondpre-charge signal during the second pre-charge stage.
 11. The displaydevice of claim 1, wherein the pre-charge current has a constantamplitude which is higher than a driving current of the least one ofsaid elements.
 12. The display device of claim 11, wherein thepre-charge voltage initially increases the driving current, the drivingcurrent decreasing to less than the driving current while the pre-chargevoltage is applied.
 13. The display device of claim 1, wherein thepre-charge current is decreased when a threshold voltage is reached, thethreshold voltage being less than an operating voltage of the least oneof said elements.
 14. The display device of claim 7 wherein, as thepre-charge current decreases, the means for selecting selects a lowerresistance so that a higher current is obtained for faster charging.